Soft Clay Foundation: Bearing Capacity Passes but Settlement Governs
A four-storey RC office building is to be founded on a 6 m thick normally consolidated soft clay layer. Undrained shear strength is 60 kPa, coefficient of volume compressibility is 4 × 10⁻⁴ m²/kN, bulk unit weight is 18 kN/m³, and the water table lies 2.0 m below ground level. The foundation is a 2.5 m × 2.5 m isolated pad founded at 1.5 m depth, carrying a service column load of 800 kN. The permissible total settlement for the structure is 40 mm. The structural engineer wants the foundation finalised this week and has asked two questions: whether the pad as designed is acceptable, and whether simply enlarging the pad to 3.0 m × 3.0 m will solve any problem. The borehole terminates at 12 m depth without meeting rock or any stiff stratum.
1. Check the net safe bearing capacity of the 2.5 m × 2.5 m pad using Skempton's approach with a factor of safety of 2.5, and compare it with the applied net pressure.
Read the worked answer
Ultimate net bearing capacity (Skempton, square footing, D/B = 1.5/2.5 = 0.6):
q_nu = 5 c_u (1 + 0.2 B/L)(1 + 0.2 D/B) = 5 × 60 × 1.2 × 1.12 = 403 kPa
If the simplified form without the depth term is used, q_nu = 5 × 60 × 1.2 = 360 kPa. Taking the conservative 360 kPa:
q_net,safe = 360/2.5 = 144 kPa
Applied net pressure from the column = 800/(2.5 × 2.5) = 128 kPa
The pad is therefore adequate in bearing, with a utilisation of 128/144 = 89%. Note that the water table (0.5 m below the base) does not materially change the net capacity in undrained terms, because the failure mechanism is governed by c_u rather than by effective weight - but it does matter for the construction of the excavation and for any dewatering.
A single check would wrongly conclude that the foundation is settled. Bearing capacity is not the governing criterion in soft normally consolidated clay; settlement almost always is, and must be checked before the pad is confirmed.
2. Estimate the consolidation settlement of the 6 m clay layer under the 800 kN load and compare it with the permissible 40 mm. Would enlarging the pad to 3.0 m × 3.0 m be an effective remedy?
Read the worked answer
Stress increase: using 2:1 load spread, at the mid-depth of the clay layer (3.0 m below the base):
Δσ = 800/(2.5 + 3.0)² = 800/30.25 = 26.4 kPa
Settlement: ρ = m_v Δσ H = 4 × 10⁻⁴ × 26.4 × 6000 = 63.5 mm, against a permissible 40 mm.
The foundation fails on serviceability by a wide margin - 1.6 times the limit.
Enlarging to 3.0 m × 3.0 m: Δσ = 800/(3.0 + 3.0)² = 800/36 = 22.2 kPa, giving ρ = 4 × 10⁻⁴ × 22.2 × 6000 = 53.3 mm - still failing. Even a 4.0 m square pad gives 800/(7.0)² = 16.3 kPa and 39.2 mm, only just inside the limit, at 2.6 times the concrete area and a much larger excavation.
Enlargement is therefore technically ineffective and economically poor, because the stress bulb in a 6 m clay layer is deep and the load spreads only as 1/(B + z)². The correct reading is that settlement, not pressure, controls the foundation type. One caution to record: the 2:1 distribution is an approximation; a Boussinesq or influence-chart check at the layer centre would normally give a slightly different value, but not enough to change the conclusion.
3. Recommend a foundation solution, with justification, and state what additional information you would require before issuing the design.
Read the worked answer
Since bearing is satisfied and settlement controls, the options must be judged on their ability to reduce settlement, not increase capacity:
- Piles with the pad acting as a cap. The 12 m borehole has not proved rock, so end bearing cannot be claimed. Friction piles would have to develop capacity in the soft clay (α c_u, with α ≈ 0.4–0.5), which is poor; a piled solution is credible only if a stiffer stratum can be proved below 12 m. Deepen the borehole with an SPT/CPT before committing.
- Ground improvement by prefabricated vertical drains with staged surcharge preloading. Well suited to a 6 m normally consolidated clay: most of the 63 mm is primary consolidation that can be removed before construction. It requires a time window (typically 6–12 months) and settlement monitoring; secondary compression must be added to the residual settlement.
- Raft. Only sensible if the whole building footprint is loaded, converting isolated pads into one raft and reducing differential settlement; it does not by itself reduce total settlement much.
- Reduce load or accept settlement with a flexible services connection - rarely defensible against a 40 mm limit.
Recommendation: deepen the investigation and go to PVD plus surcharge, with piles as the fallback if a competent stratum is found. Before issuing, require: undrained triaxial tests on undisturbed samples at several depths, c_v and m_v from oedometer tests, confirmation that the clay is normally consolidated, a check on secondary consolidation, and a plate load or full-scale trial if available. The settlement estimate of 63 mm should also be reworked with a proper stress distribution and a layered summation rather than a single m_v for the full thickness.